Multi-junction photovoltaic cells provide a method for increasing the efficiency of solar cells and panels and other devices that produce or use electrical energy from light. In multi-junction photovoltaic cells, each junction has a band gap attuned to capture a specific range of photon energies in the visible spectrum. Generally, each junction in a cell has a different band gap in order to broaden the range of photon energies which a cell is able to capture. This helps to increase the overall efficiency of the cell.
Photovoltaic cells produce peak power at only one current and voltage operating point. This one operating point is known as the maximum power point (MPP). Operation at any other point reduces the output power and hence cell efficiency. However, in a multiple junction series connected cell, operating every junction at its respective peak power point is difficult because each junction will have a different MPP due to the differing band gaps of the junctions.
Placing the junctions in series dictates that the same current flow through successive junctions. Consequently, where this current does not match the MPP of a junction through which it is flowing, the efficiency of that junction will be compromised. By preventing one or more of the junctions from operating at its MPP, the overall power output and efficiency of the cell is lower than if all the junctions were operated at their MPPs.
The junctions could be connected in parallel in an attempt to operate more of the junctions at their respective MPPs. However, parallel connections result in a lower efficient for downstream power converters that aggregate and condition the electrical output of the cell. Consequently, multi-junction photovoltaic cells have not been successfully commercialized.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.